Seems I was mistaken, but it looks like you can still derive the power by looking in /sys/class/power_supply/BAT0/ and whether ../AC/online is 0.
On my laptop BAT0 a bit slow to update, with an update rate of about 0.2Hz.
I've also noted my desktop's UPS reports different power draw values on its status display and the UPS' USB interface, by about 10% or so.